Complete circulation type degradable polymers which are finally degraded into carbon dioxide and water by degradation by means of microorganisms under natural environment and/or by hydrolysis under neutral environment, attract attention recently from the viewpoint of environmental protection.
For example, among such polymers, polyhydroxybutyrate (hereinafter referred to as "PHB"), polycaprolactone (hereinafter referred to as "PCL") and polylactic acid are known as a melt-moldable and biodegradable polymer.
However, not only PHB requires a large energy in recovery and purification of the polymer because of biosynthesis by microorganisms, thus the production cost is too high, but also molding thereof is difficult because it is difficult to control the molecular weight and the crystallinity. Also, it is difficult to control the physical properties of the molded articles such that the molded articles are poor in transparency. Thus, it is the actual circumstances that it is not easy to industrially and inexpensively provide performances and moldability which meet the uses.
Also, PCL has a serious problem and an obstacle in practical use that the creep during use is large since its melting point is as low as 60.degree. C. and, therefore, the articles prepared therefrom are poor in shape stability or the strength is extremely lowered depending on the use temperature.
On the other hand, polylactic acid is relatively inexpensive, and is a thermoplastic resin having a sufficient heat resistance since its melting point is 178.degree. C. Thus, it is melt-moldable and the use as fibers for clothing and industrial purposes is expected. Also, although the polylactic acid is a biodegradable polymer excellent on practical use, it has the problems in production and processing that (1) polylactic acid homopolymer is poor in melt-moldability due to too high crystallinity, and moreover, the molded articles, films, fibers and the like obtained therefrom are not sufficient in toughness, and also are fragile and low in impact strength (having a rigid crystal structure), (2) the dye affinity is poor, (3) the molecular weight of polylactic acid cannot be sufficiently raised, (4) if polylactic acid is heated, the molecular weight is decreased, resulting in deterioration of strength and the like of the final products, and in addition, a technique to produce practical fibers for clothing and for industrial use from polylactic acid has not yet been established, (5) it is behind technical development for commercialization as fiber products.
For such a reason, a very limited use such as thread for suture utilizing the biocompatibility is only hitherto known.
Also, Japanese Patent Publication Kokai No. 1-163135 discloses a drug sustained release base material for releasing drug into living body, which is obtained by reacting a polymer or copolymer of lactic acid having a molecular weight of 300 to 10,000 with a polyoxyethylene glycol (hereinafter referred to as "PEG") having a molecular weight of 150 to 10,000 in an equivalent ratio of PEG to the polylactic acid of 0.3 to 5.0 (30 to 500%).
However, the obtained copolymer is contemplated to use mainly in a living body. Its softening point (temperature at which stringiness begins to occur with a glass bar on a hot plate) is as very low as from -10.degree. to 60.degree. C., and the molecular weight is supposed to be at most about 10,000 to about 20,000, based on the above-mentioned softening point, ratio of reaction raw materials and state of the product (paste-like or wax-like). Therefore, molded articles excellent in versatility and toughness cannot possibly be obtained therefrom.
Also, Japanese Patent Publication Kokai No. 63-69825 discloses a block copolymer of 70 to 97% (% by weight, hereinafter the same) of polylactic acid segments and 3 to 30% of. polyoxyethylene dicarboxylic acid segments. It is described therein that the reason why the polyoxyethylene dicarboxylic acid is used is that if PEG is reacted at the time of polymerization of a cyclic dimer of lactic acid (hereinafter referred to as "lactide"), the terminal hydroxyl groups of PEG hinders the polymerization, so the copolymer having a low degree of polymerization is only obtained.
However, the products obtained in the examples of Japanese Patent Publication Kokai No. 63-69825 are only those having a molecular weight of at most 31,000 and a tensile strength of the film of only 2.8 kg/mm.sup.2 (about 1/10 of the product of the present invention), even if polyoxyethylene dicarboxylic acid is used. Further, polyoxyethylene dicarboxylic acid is fairly expensive as compared with PEG and, therefore, the products are not wholly satisfactory also from the viewpoint of versatility.
Besides polymers having biodegradability as mentioned above and artificial fibers prepared therefrom, cotton, wool, silk and the like which are natural fibers also have a biodegradability, but there are limits in strength, fineness, length and the like, so the uses thereof are limited.
Also, in general, if natural fibers are retained in the soil or water rich in bacteria, the degradation progresses in about 1 to 3 months, and they have the defect that the article life is too short.
For such a reason, artificial fibers which can be produced to have a desired thickness and length and which have a rate of degradation or rate of deterioration according to necessity, have been strongly demanded.
Also, conjugate fibers which can be divided by a chemical treatment to produce, for example, ultrafine fibers, have been widely utilized. For example, conjugate fibers that the division is possible by treating with an aqueous solution of a strong alkali (sodium hydroxide and the like) to hydrolyze polyesters, are disclosed in Japanese Patent Publication Kokai No. 57-29610, No. 59-187672 and No. 1-292124. Also, conjugate fibers which can be divided by dissolving and removing a soluble polymer (e.g. polystyrene) with a solvent (hydrocarbon, polar solvent, halogenated compound or the like), are proposed in Japanese Patent Publication Kokai No. 61-282445, etc.
However, these conventional dividable conjugate fibers have many problems in safety and environmental protection, since aqueous strong alkali solutions or organic solvents are used in the division.
For example, since alkali hydrolysis uses an aqueous solution of sodium hydroxide having a high concentration (e.g. 1% or more, especially about 3 to about 10%), a large amount of an acid is required in neutralization of waste water after the hydrolysis treatment. Further, the hydrolysis products (sodium terephthalate, etc.) have a low rate of biodegradation and accordingly become a source of environmental pollution. Similarly, in case of using solvents, it is difficult to completely recover the solvents and the dissolved polymers from the waste water, resulting in source of environmental pollution. Also, if the dangerous aqueous strong alkali solutions or the solvents are used and the treatment of the waste water thereof is further conducted sufficiently, not only difficulties on working are encountered, but also expensive equipments and high operating cost are required, thus economically disadvantageous.
It is an object of the present invention to provide a novel polylactic acid copolymer (biodegradable copolyester) which is improved in moldability and toughness and is improved in rate of degradation, impact strength and/or dye affinity and moreover has a sufficient heat resistance and which is relatively inexpensive and can be used in a wide range of uses.
A further object of the present invention is to provide molded articles prepared by melt-molding the above-mentioned polylactic acid copolymer (for example, a melt-adhesive fiber of the polylactic acid copolymer having a melt-adhesion property as well as a biodegradability and suitable for preparing fiber structures having a complete circulation type biodegradability such as non-woven fabrics and woven and knit fabrics; a novel fiber which is biodegradable or is hydrolyzable under a natural environment, and that it is possible to control its life (period of use) within a wide range in accordance with necessity and to afford with a high reliability a very favourable characteristic such that deterioration in strength and physical properties is relatively less during the use and rapidly proceeds after the life; an improved novel dividable conjugate fiber which can be easily divided under neutral or weak alkaline environment and whose hydrolysis products can be easily degraded to prevent environmental pollution, and which is low in waste of resource and is advantageous also in cost and a fiber structure produced utilizing it).
Another object of the present invention is to provide a novel process for producing polylactic acid copolymer moldings having a high strength which are improved in formability and toughness and can be used in a wide range of uses at low cost.